Abstract

ZnOthin film based surface acoustic wave(SAW)devices have been utilized to fabricate microfluidic pumps. The SAWdevices were fabricated on nanocrystalline ZnOpiezoelectricthin films deposited on Si substrates using rf magnetron sputtering and use a Sezawa wave mode for effective droplet motion. The as-deposited ZnOsurface is hydrophilic, with a water contact angle of , which prevents droplet pumping. Therefore, the ZnOsurface was coated using a self-assembled monolayer of octadecyltrichlorosilane which forms a hydrophobic surface with a water contact angle of . Liquid droplets between 0.5 and in volume were successfully pumped on the hydrophobic ZnOsurface at velocities up to . Under acoustic pressure, the water droplet on an hydrophilic surface becomes deformed, and the asymmetry in the contact angle at the trailing and leading edges allow the force acting upon the droplet to be calculated. These forces, which increase with input voltage above a threshold level, are found to be in the range of . A pulsed rf signal has also been used to demonstrate precision manipulation of the liquid droplets. Furthermore, a SAWdevice structure is demonstrated in which the ZnOpiezoelectric only exists under the input and output transducers. This structure still permits pumping, while avoiding direct contact between the piezoelectric material and the fluid. This is of particular importance for biological laboratory-on-a-chip applications.

Received 11 November 2008Accepted 05 December 2008Published online 23 January 2009

Acknowledgments:

This work was supported by the IT R&D program of MIC/IITA, Republic of Korea (Grant No. 2005-S605-02, IT-BT-NT Convergence Core Technology for Advanced Optoelectronic Devices and Smart Bio/Chemical Sensors). Y.Q. Fu would like to acknowledge support from the Royal Society of Edinburgh and Carnegie Trust.